As is well known, silver is a basic component of many photographic processing liquids. In the development of most photographic films, papers and the like, the concentration of silver is automatically maintained within a predetermined range by specialized equipment. Excess silver-bearing liquid is discharged by such equipment into waste holding tanks for later disposal.
Most sewage treatment agencies regard silver suspended in photographic fixers and other liquids as a hazardous material. Thus, silver-bearing liquids must be treated to remove substantially all of their silver before the liquid may be deposited into conventional sewage conduits. The concentration of silver permitted to remain in the disposed liquid varies from locality to locality, but generally must fall below 5 parts per million. Many agencies, however, require a lower concentration of silver in a liquid destined for sewer disposal.
Systems of varied design have been proposed for the recovery of silver from photographic processing liquids and the like. In each of these systems, the silver-bearing liquid is fed through one or more silver recovery cartridges filled with a reaction medium such as a mass of iron wool, iron wire screen or iron particles. Under standard conditions, the silver ions in the liquid undergo exchange reactions with the ferrous ions of the reaction medium. The silver is, thus, precipitated from the liquid in the form of a thin coating upon the steel wool and a dense sludge that accumulates at the bottom of the silver recovery cartridge.
A principal disadvantage of the silver recovery systems currently available is that the waste holding tank is often remotely positioned from its associated silver recovery cartridge(s). The relatively long hoses utilized to connect the elements together, then, must be carefully handled to avoid their kinking or severing which may result in uncontained spills of silver-bearing liquid. The remote positioning of the holding tank from the silver recovery cartridge(s) also requires excessive floor space and presents obvious difficulties when movement of the system is required. A need, therefore, exists for a compact silver recovery system that may be readily transported and is capable of safely containing spills that may occur due to operator carelessness, etc.